1. Field of the Invention
The present invention relates to a fluorene derivative, an organic compound (an organic semiconductor material), and a light-emitting element, a light-emitting device, and an electronic device using the organic compound.
2. Description of the Related Art
An organic compound can take a variety of structures in comparison with an inorganic compound and can be used to synthesize a material having a variety of functions with appropriate molecular design. Owing to these advantages, electronics utilizing a functional organic material has been attracting attention in recent years.
As examples of electronic devices using an organic compound as a functional material, there are solar cells, light-emitting elements, organic transistors, and the like. These are devices in which electric properties and optical properties of organic compounds are utilized. In particular, tremendous progress has been made in light-emitting elements.
It is said that the light emission mechanism of a light-emitting element is as follows: by application of voltage between a pair of electrodes with a light-emitting layer interposed therebetween, electrons injected from a cathode and holes injected from an anode recombine in the luminescence center of the light-emitting layer to form molecular excitons, and when the molecular excitons relax to a ground state, energy is released to emit light. A singlet excitation state (S*) and a triplet excitation state (T*) are known as excited states. Light emission is considered possible through either singlet excitation or triplet excitation. In addition, the statistical generation ratio of the excitation state in the light-emitting element is considered to be as follows: S*:T*=1:3.
As for a compound which converts a singlet excited state to light emission (hereinafter, such a compound is referred to as a “fluorescent compound”), light emission from the triplet excited state (phosphorescence) is not observed but only light emission from the singlet excited state (fluorescence) is observed at room temperature. Therefore, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% on the basis of the statistical generation ratio, S*:T*=1:3.
In contrast, when a compound in which a triplet excited state is converted into light emission (hereinafter, such a compound is referred to as a “phosphorescent compound”) is used, the internal quantum efficiency can be theoretically 75% to 100%. In other words, emission efficiency that is 3 times to 4 times as much as that of the fluorescence compound can be achieved. For these reasons, in order to achieve a highly efficient light-emitting element, a light-emitting element using a phosphorescent compound has been actively developed recently.
When a light-emitting layer of a light-emitting element is formed using the above phosphorescent compound, in order to suppress concentration quenching of the phosphorescent compound or quenching due to triplet-triplet annihilation (T-T annihilation), the light-emitting layer is often formed so that the phosphorescent compound is dispersed in a matrix of another substance. In that case, a substance serving as a matrix is referred to as a host material, a substance that is dispersed in a matrix, such as a phosphorescent compound, is referred to as a guest material.
In the case where a phosphorescent compound is used as a guest material, a host material is required to have a large energy gap (a difference between the highest occupied molecular orbital level (HOMO level) and the lowest unoccupied molecular orbital level (HOMO level)) or triplet excitation energy (a difference in energy between a ground state and a triplet excited state) higher than that of the phosphorescent compound. Therefore, a substance having such characteristics has been developed.
For example, in Patent Document 1, a substance having a fluorene skeleton, the structural formula of which is shown below, is disclosed as a material suitable for a host material of a light-emitting element.
    [Patent Document 1] PCT International Publication No. W2005/092857